The objectives of this project are (1) to investigate radiation-induced changes in the immature (early postnatal) spinal cord, (2) to evaluate the reparative and regenerative capacities of the immature nervous system and (3) to study the manner in which ionizing radiation alters these capacities. The type of ionizing radiation used is soft x-rays and the experimental animal is the rat because the developmental status of this animal during the early postnatal period makes it possible to study the immature system ex utero. Most changes are evaluated light microscopically and autoradiogrphically. Particular areas of interest are in development of neuroglia and intramedullary Schwann cells and in reactions to injury of the cells lining the central canal. Re ponses of this immature system are compared with these reports by others in the adult state. Perhaps such comparisons will provide insight into reactions and limitations characteristic of the mature state. In addition to the experimental data, the information derived from intact, control animals will contribute to an understanding of normal maturation of the nervous system.